1. Field of the Invention
The present invention relates to a ferrite material and a LTCC (Low Temperature Cofire Ceramic) substrate having formed in its inside a ferrite layer made of the ferrite material in which is embedded a coil-shaped conductor layer.
2. Description of the Related Art
To date electronic equipment such as a mobile communication apparatus, typified by a cellular telephone, has been designed to have a multiplicity of electronic devices incorporated therein. In recent years, communication apparatuses such as a cellular telephone have been becoming smaller in size so rapidly that downsizing and slimming have been sought after in various electronic devices designed to be mounted therein. For example, there is known an LC filter constructed by disposing a coil within a glass ceramic substrate. In the LC filter, in contrast to a conventional LC filter to which a coil, or a chip component is added externally, a coil is disposed inside an insulator substrate such as a ceramic substrate, wherefore both compactness and slimness can be achieved. In particular, a coil for providing an inductance of greater than 100 nH comes under the category of relatively large-sized chip components. Therefore, the success of inclusion of such a coil into an insulator substrate contributes significantly to a compact and slimmed-down LC filter. As coils designed to be incorporated in a ceramic substrate, there are known a solenoid coil and a planar spiral coil. The former is constructed by connecting coil elements in the direction of thickness of a ceramic substrate. The latter is composed of coplanar coil elements. The use of a planar spiral coil is especially desirable if the ceramic substrate is made lower in profile, because it is constructed by forming coil elements on the same plane.
Such a glass ceramic substrate is usually made extremely small in size, for example it is shaped like a plate of a few square millimeters. Moreover, in order to improve the ease-of-use of the glass ceramic substrate, as well as to produce the glass ceramic substrate and other electronic devices in an efficient manner, the glass ceramic substrate is formed out of a multiple-dividable motherboard. That is, a single wide-area motherboard is used to fabricate a plurality of glass ceramic substrates concurrently and collectively. To be more specific, the substantially flat-shaped motherboard is designed to have, at least on one of its upper and lower main surfaces, division grooves formed so as to extend longitudinally and transversely, so that the motherboard can be divided into a plurality of glass ceramic substrates. The motherboard is fractured along the division grooves to obtain a plurality of glass ceramic substrates.
However, in an insulator substrate such as a ceramic substrate having a built-in coil, the coil is formed within a non-magnetic substrate. Therefore, inclusion of such a coil as allows acquisition of a relatively high inductance of approximately 100 nH cannot be achieved without increasing the number of turns in the coil. This gives rise to the impossibility of achieving compactness and slimness for the substrate in an effective manner.
According to a recently known technique devised to overcome the above stated problem, a ferromagnetic ferrite layer is formed within a glass ceramic substrate, and a coil is buried in the ferromagnetic ferrite layer. In this way, a coil for providing an inductance of greater than 100 nH can be incorporated in the substrate without increasing the number of turns in the coil. This makes it possible to simplify a surface-mounting operation, as well as to achieve miniaturization for the glass ceramic substrate.
Note that, in fabricating such a glass ceramic substrate, in order for the ferrite layer and the glass ceramic insulating layer to be firmly bonded to each other by exploiting the diffusion of a glass element of the glass ceramic insulating layer in the ferrite layer, the ferrite layer and the glass ceramic insulating layer are fired at the same time.
Related art techniques are disclosed in Japanese Unexamined Patent Publications JP-A 2-101714 (1990), JP-A 6-20839 (1994), JP-A 6-21264 (1994), and JP-A 6-333743 (1994).
However, a glass ceramic substrate having a ferrite layer of conventional design has encountered the following problems.
The first problem is the low magnetic permeability of the ferrite layer interposed between the glass ceramic layers. In ordinary cases, the ferrite layer is fired at a temperature of greater than 1000° C. In the conventional construction, however, the ferrite layer is fired at a temperature in a range of from 800° C. to 1000° C., which is a firing temperature range set for the glass ceramic substrate. This necessitates adding a sintering aid to the ferrite layer, such as glass powder, SiO2, and Al2O3. In general, the magnetic characteristics of a magnetic material such as ferrite are expressed in magnetic permeability (μ) as an indication. If the magnetic material exhibits high magnetic permeability, a coil of high inductance can be used. However, in the presence of a non-magnetic substance, the magnetic material undergoes a decrease in magnetic permeability. The decrease of the magnetic permeability is proportional to the cube of the volume of the non-magnetic substance. In other words, the addition of the non-magnetic substance such as glass powder or other sintering aid to the ferrite layer results in creating a non-magnetic area in the ferrite layer. As a consequence, the ferrite density in the ferrite layer is decreased and this leads to poor magnetic permeability.
Furthermore, in the case of firing the ferrite layer and the glass ceramic insulating layer at one time, because of the significant difference in thermal expansion coefficient between the ferrite layer and the glass ceramic insulating layer, the ferrite layer is subjected to a stress in the course of firing, in consequence whereof there results magnetic distortion. This leads to an undesirable decrease in the magnetic permeability of the ferrite layer.
The second problem is spalling of the ferrite layer. That is, when the ferrite layer is increased in thickness to obtain a sufficiently high inductance, due to the stress resulting from the difference in thermal expansion coefficient between the ferrite layer and the glass ceramic substrate, the ferrite layer is prone to come off after the completion of firing.
The third problem is roughening of the glass ceramic substrate due to inadequate sintering. That is, in the case of firing the glass ceramic insulating layer and the ferrite layer of the glass ceramic substrate at the same time, the glass element of the ferrite layer and the glass element of the glass ceramic substrate are bound to each other, and thereby the ferrite layer is inhibited from shrinking under restraint of the glass ceramic insulating layer. As a consequence, the ferrite layer lying in the inside of the glass ceramic substrate cannot be sintered properly, thus causing undesired roughening. In this case, where the glass ceramic substrate is formed out of a multiple-dividable motherboard, since it is obtained by dividing the glass ceramic motherboard into pieces after the completion of firing, it follows that the roughened part resulting from inadequate sintering in the ferrite layer is exposed from the side surface of each individual glass ceramic substrate. If, for example, atmospheric moisture finds its way into the exposed part, the glass ceramic substrate becomes water-absorptive, which leads to deterioration of the electrical characteristics of the glass ceramic substrate, such as occurrence of a short circuit in the wiring conductor disposed in the inner layer of the glass ceramic substrate. After all, the glass ceramic substrate offers poor electrical reliability.
Meanwhile, ceramic substrates can be produced without using glass ceramic. For example, a plurality of ceramic layers are formed of non-magnetic ferrite. Then, a ferromagnetic ferrite layer is interposed in the ceramic layers and a coil is buried in the ferromagnetic ferrite layer, whereupon a ceramic substrate is fabricated.
The interposition of the magnetic ferrite layer in the non-magnetic ferrite layers is necessary for the following reason. In a case where a wiring is disposed directly on the surface of the ferrite layer, a high inductance is induced in the wiring, thus causing noise. This may possibly give rise to a circuitry malfunction. Accordingly, by forming a coil in the magnetic ferrite layer and disposing a wiring in the non-magnetic ferrite layer, it is possible to reduce the inductance of the wiring, and thereby avoid occurrence of a circuitry malfunction. However, the use of non-magnetic ferrite poses the following problems.
Firstly, in a conventional ceramic substrate constructed by forming a ferrite layer and a coil-shaped conductor layer inside a non-magnetic ferrite substrate, magnetic lines of force developed in the coil-shaped conductor layer are caused to radiate toward the exterior of the ceramic substrate. Because of the instability of the magnetic lines of force, the ferrite layer is liable to undergo magnetic saturation caused by a disturbance of the magnetic lines of force. Thus, upon application of a large current, there occurs a decrease in inductance; that is, the so-called superposition characteristics are impaired. Furthermore, the magnetic lines of force radiating toward the exterior of the ceramic substrate are liable to have an electrical influence on the semiconductor chip, chip component, and wiring mounted on the upper and lower surfaces of the ceramic substrate, thus causing a circuitry malfunction.
In order to prevent the magnetic lines of force developed in the coil-shaped conductor layer from electrically affecting the semiconductor chip, chip component, and wiring mounted on the upper and lower surfaces of the ceramic substrate, it is necessary to secure a sufficient distance between the coil-shaped conductor layer and the semiconductor chip, chip component, wiring. As a result, the ceramic substrate is increased in thickness as a whole, thus making it impossible to realize a slimmed-down substrate.
Secondly, in the conventional ceramic substrate constructed by forming a ferrite layer and a coil-shaped conductor layer inside a non-magnetic ferrite substrate, in a case where metallized wiring layers are formed on the surface or in the inside of the substrate, if the thickness of the non-magnetic ferrite layer constituting the insulator substrate is reduced to make the substrate smaller in size and lower in profile, a leakage current will occur between the metallized wiring layers. This is because the non-magnetic ferrite layer has a relatively low volume resistivity value of 1×1010 Ωm or below.